In vitro assays that provide an accurate and sensitive measure of the potency of inactivated poliovirus vaccines are currently being developed. An exnzyme liked immunoassay (ELISA) using monoclonal antibodies specific for type and virus conformation has provided both accurate and consistent results. Analysis of vaccine samples has confirmed the utility of the ELISA procedures using monoclonal antibodies versus polyclonal antibodies for quantitating the viral antigen. Improvements in the ELISA procedure are in progress that will increase the sensitivity of the assay to the antigen.